1. Field of the Invention
This invention relates to a pneumatic tire, and more particularly to a pneumatic tire in which the structure of the carcass cord layer is improved to minimize partial or biased wear of the tire while improving its straight running stability, by reducing the ply steer in a radial tire for truck and bus or for small truck in which a belt-reinforcing layer consisting of a transient-reinforcing layer and at least two belt tension-resistant layers is overlaid and interposed between a tread and a carcass cord layer.
2. Description of Prior Art
A conventional radial tire for truck and bus or for small truck has a construction in which, as illustrated in FIG. 3 of the accompanying drawings, a belt-reinforcing layer is overlaid and interposed between a tread and a carcass cord layer, the belt-reinforcing layer consisting of a transient-reinforcing layer positioned in contact with the carcass cord layer with its reinforcing cords disposed at an angle of between 40.degree. to 75.degree. with respect to the circumferential direction of the tire, and at least two belt tension-resistant layers in which the reinforcing cords are disposed at angles of between 15.degree. to 30.degree. and 150.degree. to 165.degree., respectively, with respect to the circumferential direction of the tire so as to cross each other, and the carcass cord layer consists of one or more layers in which the reinforcing cords are arranged at an angle of about 90.degree. with respect to the circumferential direction of the tire. A radial tire of this kind is superior to a bias tire in brake performance, low fuel consumption, and wear resistance, thanks to the belt-reinforcing layer; but the problem with such a radial tire is that its straight running stability is poor because of the belt-reinforcing layer. As the radial tire rotates and moves forward, a lateral force occurs either to the right or left of the forward direction, even if the slip angle is zero. This lateral force makes the vehicle move in a direction different to that chosen by the driver.
In general, the lateral force when the slip angle is zero consists of force components generated by two different mechanisms, one is referred to as "conicity" (CT) and the other "ply steer"0 (PS). They are classified as part of the uniformity characteristics of the tire. The conicity CT and ply steer PS can be expressed by the following formulas, from their definition in accordance with the uniformity testing method (JASO C607) for car tires: EQU LFDw=PS+CT (1) EQU LFDs=PS-CT (2)
where LFD is the mean value of the lateral forces experienced while the tire rotates once, LFDw is the mean value measured at the outer side of the tire, and LFDs is the mean value measured when the tire is turned inside out. PS and CT can be given as follows from formulae (1) and (2): EQU CT=1/2(LFDw-LFDs) (3) EQU PS=1/2(LFDw+LFDs) (4)
The relationships of formulas (1) through (4) can be illustrated diagrammatically as in FIG. 1.
Of the conicity and ply steer described above, conicity is believed to be a force generated because the tire shape is geometrically asymmetric about the center of the circumferential direction of the tire, that is, a force generated when a tire in the shape of a truncated cone rolls. The main reason for the generation of this force is the influence of the positions of belt-reinforcing layers inserted into the tread of the tire. Hence the force can be reduced by improving the manufacture procedure. In contrast, ply steer is a force inherent to and arising from the structure of the belt-reinforcing layers themselves, and hence can not be reduced much in practice unless the structure of the belt-reinforcing layers is changed.
Now, let us consider a belt-reinforcing layer. It can be represented as a three-layer laminated sheet 50 consisting of two belt tension-resistant layers 50u, 50d and a belt-strengthening layer 50s, as shown in FIG. 2(A). It is well known that when a tensile force acts upon this three-layer laminated sheet 50 in the circumferential direction EE' of the tire, the three-layer laminated sheet 50 undergoes deformation, not only in the two-dimensional plane in which the tensile force acts, but also three-dimensionally outside the plane, so that torsional deformation like that shown in FIG. 2(B) occurs. The ply steer mentioned above occurs because of the torsional deformation of the belt-reinforcing layers.
In the past, various studies have been done to determine whether this ply steer could be reduced by adding an extra belt-reinforcing layer or layers to the existing belt-reinforcing layers. The addition of extra belt-reinforcing layers is not preferable because it would adversely affect the characteristics of the radial tire such as its low fuel consumption characteristics.